I've used a Singh-Ray Vari-ND for a few years, and I like the convenience; indispensable for bright light waterfalls/waves...still, a 77mm was expensive, 16-35 & 24 TS-E lenses are 82mm, and ultra-wides often (usually?) require a filter holder because of bulging elements...so, simplicity can be limiting. I don't shoot much really wide, so I haven't re-invested in holders since I switched from film, and only occasionally miss the ol' bag-o-filters...
Jerry
I have both but find the fixed one is what I carry, the Sing Ray does not go wide enough for my taste. You also have to deal with this cross polarization look on blue skies, totally unusable, so I mostly pack the B+W 10 stop ND.